1. Field of the Invention
The present invention relates to a polarizing optical device and a liquid crystal display (LCD) using the same.
2. Description of Related Art
A LCD is an apparatus that changes light emitted from a backlight unit (BLU) into linearly polarized light using a polarizer, rotates the linearly polarized light according to the application of voltage to a liquid crystal (LC) layer, and selectively transmits the polarized light using an analyzer, thereby modulating the polarized light and realizing black and white display. At the present time, color display may also be accomplished using a color filter.
A typical polarizer is a dichroic polarizer manufactured by dyeing oriented poly vinyl alcohol (PVA) film with a compound (urea or dichroic dye) having light absorption anisotropy. The polarizer absorbs a polarized light component parallel with an absorption axis of the dichroic dye and transmits a polarized light component perpendicular to the absorption axis of the dichroic dye. Only light transmitted through the polarizer is used as the linearly polarized light. The dichroic polarizer is an absorption type polarizer, having a natural light transmittance of about 50% or less. Consequently, the optical efficiency of the dichroic polarizer is very low.
In order to prevent loss due to the optical absorption of the dichroic polarizer, a reflection type polarizer may be employed, which is capable of reflecting linearly polarized light having a specific direction and transmitting another polarized light having a different direction.
As an example of a reflection type polarizer, there has been proposed a polarizer having an extrafine metal grid (wire grid) structure. Specifically, the reflection type polarizer may be constructed of metal wires having a diameter much less than the wavelength of light to be displayed. The metal wires may be arranged parallel to one another. Consequently, the reflection type polarizer may have polarization characteristics in which the reflection type polarizer reflects a polarized light component parallel to the metal wires, i.e., a transverse electric (TE) polarized light, and transmits a polarized light component perpendicular to the metal wires, i.e., a transverse magnetic (TM) polarized light.
When the reflection type polarizer is used in a LCD, only the TM polarized light component in the non-polarized light emitted from the BLU may be transmitted through the reflection type polarizer to be incident on a LC layer. Thus, the TM polarized light may be used for display.
On the other hand, the TE polarized light may be reflected by the reflection type polarizer, thus returning to the BLU. The TE polarized light may be reflected diffusely inside the BLU. As a result of the diffuse reflection, the TE polarized light may be changed into non-polarized light. This non-polarized light may be incident again on the reflection type polarizer, which is repeatedly performed. Consequently, most of the light emitted from the BLU may be changed into TM polarized light, which is incident on the LC layer, thereby considerably improving optical efficiency of the LCD versus using a dichroic polarizer.
When a dark display operation is performed on the LCD, however, a TE polarized light component in ambient light incident on the LCD may be reflected by the reflection type polarizer. As a result, the display contrast may be decreased.
To minimize reflection of ambient light, an extrafine metal grid (wire grid) structure may be combined with an optical absorption type extrafine grid structure. However, such a combination may require a large number of materials and complicated processes. Thus, such a combination may not be practically manufactured with a size suitable for a large LCD, e.g., an LCD having a diagonal size of 20 inches or more.